As the newborn adapts to extrauterine life, important alterations occur in the cardiovascular system. The fetal ventricles work in parallel, ejecting blood at similar pressure. At birth, systemic resistance increases, while pulmonary vascular resistance decreases in the first days of life. In association with these hemodynamic alterations, changes occur in ventricular morphology. At birth the size, shape and wall thickness of the ventricles is similar. With maturation, the left ventricle enlarges, but retains its cylindrical thick-walled structure. In contrast, the right ventricle develops a crescent shape, while the lateral wall becomes thinner with a greater radius of curvature. We propose to examine ventricular structure and function in the fetal and newborn lamb, and to contrast differences between left and right ventricles at different stages of maturation. Experiments will be carried out in fetal and newborn lambs up to 50 days of age. Two preparations will be used: a heart-lung preparation and a chronic intact preparation. Ventricular function will be characterized in terms of stroke volume, stroke work power, dp/dt/kp and mean ejection rate for each ventricle. Systolic time intervals to be measured include: pre-ejection period, electromechanical interval, isovolumic period, and ejection period. Ventricular morphology will be evaluated by measurement of ventricular weight, volume, chamber size and wall thickness, and microscopic determination of fiber layer orientation, cell and sarcomere length. The significance of the research lies in its application to the newborn with a cardiac malformation. Maturational changes in ventricular structure and function may profoundly influence the direction and magnitude of shunting in various types of congenital cardiac disease.